A rotary encoder, also called a shaft encoder, is an electromechanical device used to convert the angular position of a shaft or axle to a digital code. As such, it is a type of transducer, as it transforms information about physical positioning to an electronic signal. These devices are used in robotics, in high-quality photographic lenses, in computer input devices (such as optomechanical mice and trackballs), and in rotating radar platforms.
A shaft encoder/transducer is also commonly used in oil and gas drilling, to assist with determination of position or depth in a drilling operation. A drawworks is commonly used to raise and lower a traveling block, from which is suspended a top drive, the drill string and the bottom hole assembly. It is important in many circumstances to know depth of the bottom hole assembly (BHA). Drilling line is spooled on the drawworks. From the drawworks, the drilling line extends to the crown blocks, which are located at the top of the derrick, and then down to the traveling block. The drilling line is passed several times between the traveling blocks and the crown blocks and then fastened to a fixed point called the dead-man anchor. The driller controls the drawworks, which, via the pulley system, controls the position of the traveling block in the derrick. To measure the movement of the traveling blocks, a drawworks encoder (DWE) is mounted on the shaft of the drawworks. One revolution of the drawworks will pay out a certain amount of drilling line and, in turn, move the traveling blocks a certain distance. Calibration of the movement of the traveling block to the revolutions of the drawworks is required.
Small errors in positional encoding may be magnified along the drilling line and the traveling block. It is desirable to be as accurate as possible with a drawworks encoder.